Astronomy and paleontology have something in common: we both look at old things, and try to figure out how they came to be.

In astronomy, sadly, we cannot hold the equivalent of an old bone in our hands. All we get is light from an object (barring the occasional meteorite). So we have to look very carefully to see what’s what. Sometimes, we have to look outside the visible spectrum, too. Not only that, like any good detective we have to check the context — the neighborhood — too.

Spitzer infrared image of Cas A, now with blobby goodness.

Cas A is a supernova remnant, the expanding cloud of debris from a star that blew up. In this case, the doomed star exploded in 1680. Even though it was close enough to be easily visible to the unaided eye (it should have been bright enough, barely, to see during the day!) no one noticed it. That’s because between us and the star is a thick layer of interstellar dust. This floating junk absorbs light, so the supernova was dimmed to invisibility.

That might sound like a disadvantage, but in this case, it worked out pretty well. That is, it worked out for us 300 years later. That dust has provided an important clue to the explosion itself.

The picture above is from NASA’s Spitzer Space Telescope, which snaps infrared portraits of the sky. The glowy thing at the bottom right is Cas A, the supernova remnant. The pretty cirrus stuff littering the image is dust: sheets, ribbons, and layers of it. You can see why Cas A wasn’t visible when it blew up! All that junk blocked our view.

But if you click the picture and see it properly embiggened, you’ll notice some bright blobs helpfully circled to make it easy to spot them. By looking at the region in multiple wavelengths (different "colors" of infrared light), the temperature of the dust can be determined (dust is brighter at different wavelengths depending on the temperature). Those circled blobs are much warmer than their surroundings: they range from -120 to -170 Celsius (trust me, that’s warm for dust). What gives?

Those blobs were heated by the supernova blast itself. Even though the supernova was about 160 light years from those blobs, it still was able to warm them up. That’s amazing! But this next part is totally cool. Literally.

Blobs of dust should cool pretty quickly. So why do we see them, 320 years after the supernova? It’s because we’re seeing light echoes.

Picture this: the blobs are on the other side of the supernova from our point of view. When the star blew up, the ultraviolet and X-ray flash from the event expanded outward in a sphere. 160 years later, it touched those blobs, heating them up. They in turn started emitting infrared light, and that light took another 160 years to get back to the supernova. So, light emitted by the supernova 320 years after the explosion would start its journey toward Earth just as the first infrared light from the blobs caught up to the supernova.

What we see on Earth is the blobs lighting up 320 years after the light form the supernova reached us. So while, to us, the supernova is three centuries old, we see the blobs just lighting up now!

There’s our fossil that we can examine. It’s like having a time machine, allowing us to probe the supernova at different stages of its life all at once.

I know that’s a bit confusing, but you’ve seen this effect many times. If you see a basketball player bouncing a ball from a distance, you hear the ball smack the pavement a fraction of second after you see it. But now put a wall behind the player. The sound hits that wall and reflects — echoes — back to you, delayed because that sound has to travel from the ball to the wall, then back again that distance. If it takes the sound one second to reach the wall, say, then you hear the sound delayed by two seconds. It has to make a round trip.

Same thing with Cas A. The light left the supernova, hit the blobs, then came back, adding 300+ years to the travel time. So we see them now as if the light were just hitting them!

We’re catching an echo of the original explosion, revealed as if it were fresh once again. And by measuring the dust’s properties, it can be determined that they were indeed irradiated by a flood of ultraviolet and X-ray photons, created when an immense shock wave broke through the surface of the exploding star. This "UV breakout" from the shock wave was well known to exist, but it has only been seen directly recently. This new observation gives us indirect evidence of it too.

That’s fine by me, evidence is evidence, and catching the moment a supernova is born is incredibly rare. Those little blobs have opened a new door on studying supernovae, and, as a bonus, it’s on a pretty famous one. Cas A is very well-studied, which means we have lots of other pieces of the puzzle to looks at.

That makes it all fit better, and that’s part of what science is about.

I apologize that this is off topic, but I don’t really want to subscribe to discovery so I can email you.

I would LOVE for you to appear on the Adam Carolla Show. He’s a devout atheist, albeit a layman. He doesn’t know anything about why a flagellum is not irreducibly complex, or anything about geology. He often shows his outrage about how religious people make laws that hamper science in random ways.

He’s about as educated as Seth Rogan, though on the opposite side.

I’d love to hear you on his show. You may be able to interest him in your expertise in debunking stuff like the recent bigfoot thing. He likes talking to folks who can explain that kind of stuff to average folks. He’s also smart enough to be interested in other skeptical topics like the moon hoax, etc.

He’s a morning DJ so he has lots of babel, but I think he’d be very willing to make you a regular guest. He took Howard Stern’s spot for radio and there are a alot of people who podcast his show thanls to his post on “Loveline”.

Malachi: wow, thanks. I didn’t know he had a show. I think he’s very funny, and from listening to him on Loveline (that show was pretty interesting) I think he tends to be pretty solid in his reasoning.

Oh, and Nicole: we’ve seen light echoes around lots of supernovae. A big part of my PhD thesis was dependent on the light echoes from a ring of gas around SN 1987A. Someday I’ll write more about that. It’s tremendously cool stuff.

In this case, the doomed star exploded in 1680. Even though it was close enough to be easily visible to the unaided eye (it should have been bright enough, barely, to see during the day!) no one noticed it.

Extract from Wikipedia — Cassiopeia A: “Astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova on August 16, 1680, when he catalogued a star near its position.”

Seeing images like this of beautiful, exciting, far-away places always makes me a little sad we’re in such a boring part of the universe. No glowing dust-clouds in our night skies, no stellar explosions casting waves of radiant energy across the solar system. On the other hand, if we were in the middle of it, we’d have a harder time observing all of these amazing things in the first place!

Light echoes make a great argument to use against all those “young Universe” creationists who argue the speed of light has changed over time. Not this one particularly, but the echoes from that 1987 supernova are much further away and demonstrate the speed of light was exactly the same a long time ago.

Fortunately I don’t meet many of these misguided folk so I haven’t yet had the opportunity to use it.

The Tweet: “When I was a kid, our footie pajamas weren’t flame retardant. DAMN, we were hardcore.”

The Twitterer: WilW, known in real life as the actor-turned-author Wil Wheaton. Many of you may recognize him best from his role as Wesley Crusher on “Star Trek: The Next Generation.”

Tweet Origin: “The cover of my newest book has a picture of me wearing footie pajamas,” he says (crafty plug there, Wil). “And I was looking at the cover of the book,” when he wrote the Tweet.

FYI, Wheaton is the father of two children, both of whom wear flame-retardant PJs, which makes him especially qualified to comment on the hard-coreness of today’s youth.

His Chances: Stellar. True, the “Funniest Tweets” category actually contains some much better entries (“Bozo the Clown has died. Or, for those of us who fear clowns, Bozo the ghost is born”). But Wheaton brings star power to the contest. Read: 16,778 fans who follow his Twitter feed in the hopes of some steamy updates about Trek hottie Deanna Troi.

[Bold added.]

Any discrepancies between PP’s stated reasons for reading WW’s texts and Washington Post’s claimed reasons to do the same remain to be explained.

This is definitely a cool image and goes to show how much raw power is contained in a supernova. Any word on what Cas A became after the supernova? A dwarf, a neutron, a blackhole? (not sure of its approximate size so I can’t really make an educated guess on that)

I think my favourite example of light echoes is V838 Monocerotis, which is a fascinating object anyway because it is not entirely clear what caused its outburst in the first place: one possibility is it was caused by a stellar collision.

What I don’t get about this article is this. If the blobs are 160 light-years behind Cassiopeia A doesn’t that mean there is an extra 160 light-years worth of dust and debris that light has to travel through? Wouldn’t that cause them to be much dimmer and perhaps invisible? I’m not sure why there are separate blobs either. If the supernova explosion was roughly spherical and you take a plane 160 ly behind it and intersect the sphere, you get a circle. So why isn’t there a bright circle centered on Cas A?

It’s because were looking at infrared light, which can get through the dust. If we had Spitzer in 1680, we could have viewed the supernova directly in the infrared. It’s not spherical because the blobs aren’t evenly distributed.

“Wow, you wouldn’t think mere dust would be so reflective as to allow light to “survive” that much time and distance without diffusing into nothing.”

I don’t think they’re talking about actual reflection of either visible light or infrared. Instead, if I read the original post correctly it was the X-rays, gamma rays, etc. from the initial supernova explosion going away from us and hitting gas, dust 160 light-years behind it, heating it up, and then causing it to glow in the infrared because it’s hot. Of course, for us to see the infrared glow that electromagnetic radiation must pass through 160 light-years of dust and gas on the far side and presumably another 160 light-years of dust and gas on the near side before it gets to us. So it’s surprising it’s as bright as it is. That’s why I’m skeptical of that particular explanation.

I admit I have no idea how long gamma or x-rays can maintain their coherence over that kind of distance. Neither do I know much about the reflective abilities of electromagnetic radiation in general. Is it even possible?

That dust must be toast by now, though. Hm…I’m seeing the announcement of a new particle on the horizon– toasty-dust!

O.K. I did a little bit more reading and I think I’ve got this one figured out. According to Wikipedia the actual supernova remnant is about 10 light-years across (i.e., radius = 5 light-years). So 160 light-years is way beyond the wavefront of the expanding debris. The hot blobs 160 light-years behind the supernova are part of a nebula whose structure has NOTHING to do with the supernova explosion, it was pre-existing. That’s why there doesn’t have to be a ring of hot IR emission centered around the supernova source.

It also explains why the blobs may appear so bright to us. The nebula is behind the supernova and the hot blobs are those points where the spherical wavefront of X-rays/gamma rays/etc. intersect the nebula. There does not have to be a lot of gas and dust between us and the blobs. It could be relatively empty interstellar space which explains why the blobs appear so bright to us.